The invention relates generally to fiber Bragg grating based sensing devices and, more particularly, to thermally stabilized fiber Bragg grating based sensing devices that can be operated at elevated temperatures as compared with conventional fiber Bragg grating based sensing devices.
In general, there are several techniques used for measurement of parameters such as temperatures. Some of the commonly used systems include thermocouples and pyrometry and blackbody measurement devices. A fiber Bragg grating (FBG) based fiberoptic temperature sensor includes a FBG that is a high quality reflector constructed in an optical fiber that reflects particular wavelengths of light and transmits other wavelengths. This is generally achieved by adding a periodic variation to a refractive index of the fiber. It is advantageous to use FBG sensors for power generation industrial process monitoring because of the sensors' low mass, high sensitivity, and electromagnetic interference immunity, for example.
However, conventional ultraviolet (UV) light induced FBG sensors exhibit undesirable thermal instability at elevated temperatures. UV inscribed FBGs may be of various types with several including Type I, Type IIA, and Type II. The type typically refers to the method by which gratings are produced in the fiber. The different methods of forming the gratings effect physical attributes of the gratings such as ability to withstand elevated temperatures. The fibers on which Bragg gratings are formed as well as any associated claddings of such fibers may be doped or un-doped. In some embodiments, both the cladding and core are doped. When the fiber core has no dopant, the fiber cladding is typically doped for reducing the cladding index of refraction so that the cladding can confine light wave propagation inside the fiber core. Typical doping atoms include as phosphorus, boron, fluorine, erbium, yttrium, aluminum, and tin.
Type I gratings are standard gratings written in both hydrogenated and non-hydrogenated fibers and are the only types of gratings that are commercially (off-the-shelf) available. A Type I grating is a periodic refractive index modulated grating structure and starts to degrade at temperatures higher than about 300 degrees Celsius after only a few, for example 2-4, hours of operation. Thus, it is difficult to use a Type I FBG as a sensor higher elevated temperature environments.
Type IIA gratings are regenerated gratings that are written after erasure of a Type I grating. These gratings require an additional writing step and higher energy levels of the inscribing lasers. Type IIA gratings generally have higher erasure temperatures than Type I gratings but require higher laser energy and longer inscription time. A Type II grating is a damage written grating inscribed by high power pulsed lasers. Fringes take the form of physical changes in the crystal lattice. On the other hand, Type II gratings, inscribed at high power levels, have a broad reflective spectrum that is generally undesirable for high temperature sensing applications. The high power pulses required of Type IIA and Type II gratings are achievable only with expensive processing equipment.
Therefore, a need exists for a high-temperature operable Type I FBG based fiber optic sensor and fabrication method that addresses one or more of the problems set forth above.